Answer: If you convert from A minor to C major you're of course changing mode - it's a different world. Some things just don't translate - what you're really doing is making a variation on the melody and harmony.

But a logical way to proceed is to think in terms of Roman numerals. Keep the same numerals, but use the chord as it comes out in the new scale.

What you did with your first example was to translate V7 in A minor to V7 in C: E7 to G7. No problem there.

Then you translated i in A minor to I in C: very reasonable, and likewise iv of A minor to IV of C.

What's throwing you is that G in the key of A minor is the VII chord, a whole step below the tonic. But there is no chord in C whose root is a whole step below the tonic. The most literal translation you could make would be vii° in C, which is B dim.

The other two aren't so hard when thinking in Roman numerals: C would be III in A minor, so it translates to iii in C major; F is VI in A minor, so use vi in C.

You end up with this:

The A minor version: G major, C major, F major

The C major version: B dim, e minor, a minor.

B dim to e minor is kind of funny, but I can imagine making it work.